Forcer and associated three phase linear motor system

ABSTRACT

A three phase linear motor system includes a forcer having three pole pairs, which are moveable relative to a platen. The pole pairs are arranged to oppose associated teeth of the platen. The platen teeth are spaced apart from each other in a travel direction according to a predefined tooth pitch. Each of the pole pairs includes a set of teeth that extend from magnetically coupled poles toward the platen. The set of teeth of one of the pole pairs is spaced from the sets of teeth for the other two poles by distances functionally related to about 120 degrees and 240 degrees, respectively, of the tooth pitch.

TECHNICAL FIELD

The present invention relates to motors and, more particularly, to aforcer configuration for a linear motor and to a corresponding threephase linear motor system.

BACKGROUND OF THE INVENTION

There are various configurations of linear motors, including generallyflat motors, U-channel and tubular shaped motors. Different types oflinear motors also are available, including brush, AC brushless,stepper, and induction motors. Common to most linear motors are a movingassembly, usually called a forcer, which moves relative to a stationaryplaten according to magnetic fields generated by application of currentthrough one or more associated windings. The windings can be on theforcer or at the platen depending on the type of motor.

For example, in a permanent magnet linear motor, a series of armaturewindings are mounted within a stage that is movable relative to astationary base plate or platen. The platen typically includes an arrayof permanent magnets configured to interact with the coils in the stagewhen energized with an excitation current. Alternatively, the magnetscan be located in the stage with the coils situated in the platen. Aclosed loop servo positioning system is employed to control currentthrough the windings. For example, current is commutated through coilsof the stage with a three phase sinusoidal or trapezoidal signal in aclosed loop feedback system. When such a linear motor is used in apositioning system, the relationship between the location of the stageand locations of the coils is utilized to control its operation. In sucha linear motor, the available magnetic field intensity and thus theforce is limited by the field strength of available motor magnets.

A linear stepper motor includes a forcer having windings that areinserted into a laminated core assembly. The stepper also includes astationary platen having a plurality of teeth spaced apart from eachother in a direction of movement. The forcer moves by application ofpower to a winding, which generates force by causing teeth of the forcerto align with teeth of the platen. The change in current through thewindings causes the teeth to consecutively align and, thus, createlinear motion. Because the forcer moves a predetermined amount based onthe number current pulses, a stepper motor can function as an open loopsystem that does not require servo tuning. The number of pulses tocreate motion, which varies based on the tooth pitch on the platen,determines the resolution of the movement. In order to provide desiredresolution and stiffness in a linear stepper motor, for example, platentooth pitch of about 1 mm or less is required, which typically is formedby photochemical etching.

Linear motors are increasingly being employed in manufacturingequipment. In such equipment, nominal increases in the speed ofoperation translate into significant savings in the cost of production.However, the cost of such equipment often plays a decisive role indetermining which type of system will be employed.

SUMMARY

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention. It is intended toneither identify key or critical elements of the invention nor delineatethe scope of the invention. Its sole purpose is to present some conceptsof the invention in a simplified form as a prelude to the more detaileddescription that is presented later.

One aspect of the present invention provides a forcer for a three phaselinear motor system. The forcer includes three pole pairs, each havingat least two poles that are magnetically coupled together as part of anassociated phase of the motor system. Each phase includes a coiloperatively associated with one or more of the poles, which coil can beenergized to provide an.electromagnetic field at the respective polepair. Each pole also includes a set of teeth, where each set of teethfor the respective pole pairs are offset from each other to facilitateoperation of the three phase linear motor system.

In accordance with a particular aspect, a first set of teeth for onepole pair is offset +/−120 degrees plus M*180 degrees and +/−240 degreesplus N*180 degrees, respectively, of a tooth pitch relative to the othersets of teeth for the other two pole pairs, where M and N are naturalnumbers.

Another aspect of the present invention provides a three phase linearmotor system. The motor system includes a forcer that is moveablerelative to a platen. The forcer includes three pole pairs, each polepair having windings to define an associated phase of the motor system.The pole pairs are arranged to oppose associated teeth of the platen,which platen teeth provide a return path for magnetic flux from the polepairs. The platen teeth are spaced apart from each other in a directionof movement according to a predefined tooth pitch. Each of the polepairs also includes a set of teeth, with the set of teeth of the secondpole pair being offset from the set teeth of the first pole pair byabout +/−120 degrees of the tooth pitch. The set of teeth of the thirdpole pair are offset from the set of teeth of the first pole pair by amultiple of about +/−240 degrees of the tooth pitch.

To the accomplishment of the foregoing and related ends, certainillustrative aspects of the invention are described herein in connectionwith the following description and the annexed drawings. These aspectsare indicative, however, of but a few of the various ways in which theprinciples of the invention may be employed and the present invention isintended to include all such aspects and their equivalents. Otheradvantages and novel features of the invention will become apparent fromthe following detailed description of the invention when considered inconjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a linear motor assembly in accordancewith the present invention;

FIG. 2 is a front sectional view of a linear motor illustrating a firstmotor condition, in accordance with the present invention;

FIG. 3 is view of the motor of FIG. 2, illustrating magnetic flux linesthrough the motor accordance with the present invention;

FIG. 4 is another view of the motor of FIG. 2 illustrating a secondmotor condition, in accordance with the present invention;

FIG. 5 is another view of the motor of FIG. 2 illustrating a third motorcondition, in accordance with the present invention;

FIG. 6 is another view the motor of FIG. 2 illustrating a fourth motorcondition, in accordance with the present invention;

FIG. 7 is yet another view of the motor of FIG. 2 illustrating a fifthmotor condition, in accordance with the present invention;

FIG. 8 is front view of another type of linear motor in accordance withthe present invention.

FIG. 9 is a front view of part of another linear motor arrangement inaccordance with the present invention;

FIG. 10 is a front view of part of another linear motor arrangement inaccordance with the present invention; and

FIG. 11 is an isometric view of a two-dimensional motor system inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a three phase linear motor. The motorincludes three pole pairs, each having associated windings to define anassociated phase. The pole pairs are configured to provide forcermovement relative to an associated platen. The platen teeth are spacedapart from each other in the direction of movement according to apredefined tooth pitch. Each of the pole pairs includes a set of teeth.The set of teeth of one of the pole pairs is offset about +/−120 degreesand +/−240 degrees of the tooth pitch relative to the other respectivesets of teeth for the other two pole pairs.

FIG. 1 illustrates an isometric view of a three phase linear motorsystem 10 in accordance with an aspect of the present invention. Themotor 10 includes a forcer 12 that is oriented in a spaced apartrelationship relative to a platen 14, which defines the path along whichthe forcer can traverse in a direction, indicated by arrow 16. It is tobe appreciated that the platen 14 could be configured to havesubstantially any length and any of a variety of desired configurations(e.g., a straight path, a curved path, closed loop path, etc.). It isalso to be appreciated that the forcer 12 is supported relative to theplaten 14 for movement in the direction 16, such as by low or nofriction bearings (e.g., air bearings, not shown).

In the example of FIG. 1, the portion of the illustrated platen 14includes a plurality of teeth 18 that extend substantially parallel froma planar surface 20 of the platen. The teeth 18 are spaced apart fromeach other by respective valleys 22 to provide a predefined tooth pitch,indicated at 24. Each of the teeth 18 extends longitudinally betweenside edges 26 and 28 of the platen 14. For example, each tooth 18 haswidth that approximates the width of the associated valley (e.g., toothwidth ≈½ tooth pitch). It is to be appreciated that the tooth width alsocould be slightly less than ½ the tooth pitch 24, with the valleys beingslightly greater than ½ the tooth pitch so as to mitigate cogging.

In accordance with an aspect of the present invention, the platen teeth18 are formed of non-permanent magnetic material (e.g., iron or an alloythereof) having a sufficient magnetic permeability so that the teeth aremagnetizable in response to magnetic fields from the forcer 12, suchthat the platen provides a return path for flux generated at the forcer.That is, no permanent magnets are required to be disposed on the platen14 for interaction when the forcer is energized. Accordingly,manufacturing costs associated with the linear motor system 10 can besignificantly reduced.

The forcer 12 includes three pole pairs 30, 32, and 34, each having atleast four associated teeth 36, 38, and 40, respectively, which teethare 180 degrees out of phase with each other in each pole pair. Thethree sets of teeth 36, 38, and 40 are spaced 120 degrees apart fromeach other in the direction 16 so that one or more tooth of one set ofteeth can be aligned with the platen teeth 18 during movement of theforcer 12 relative to the platen 14. For example, the 38 teeth of thesecond pole pair 32 are shifted about 120 degrees (e.g., about ⅓) of thetooth pitch 24 relative to the teeth 36 of the first pole pair 30.Similarly, the teeth 40 of the third pole pair 34 are shifted about 240degrees (e.g., about ⅔) of the tooth pitch 24 relative to the teeth 36of the first pole pair 30.

Stated more generally, the teeth of the second pole pair 32 are offsetrelative to the teeth of the first pole pair 30 by about(+/−120°+L*180°)*P/360°, where L is a natural number and P is thepredefined tooth pitch. The teeth of the third pole 34 pair are offsetin relative to the set of teeth of the second pole 32 pair by about(+/−120°+M*180°)*P/360°, where M is a natural number. The teeth of thethird pole pair are offset relative to the set of teeth of the firstpole pair 30 about (+/−240°+N*180°)*P/360°, where N is a natural number.The natural numbers L, M and N can be the same or different.

Because of such an arrangement, with every 120 degrees of movementbetween the forcer 12 and the platen 14, one set of teeth 36, 38, or 40can align with the teeth 18 of the platen.

The teeth 36, 38, and 40 of the respective pole pairs 30, 32, and 34also have a tooth width based on the tooth pitch 24 at the platen 14. Inone aspect, the tooth width at each pole pair 30, 32, 34 approximatesthe tooth width 24 of the platen teeth 18. The three phase motorconfiguration enables the platen teeth 18 and the pole teeth 36, 38, and40 to have a relatively large tooth pitch, such as, for example, a widthfrom about 5 mm to about 15. This facilitates manufacturing of the motorsystem 10, as the teeth 18, 36, 38, 40 may be formed by conventionalmachining techniques (e.g., cutting or grinding) and still provide adesired amount of resolution. This is in contrast to etching techniquestypically employed for smaller tooth widths (e.g., about 1 mm or less)that are required to obtain comparable resolution in linear steppermotors.

Each of the pole pairs 30, 32, and 34 is formed of a respective pair ofpoles 42 and 44, 46 and 48, 50 and 52. In the example of FIG. 1, thepoles 42 and 44, 46 and 48, 50 and 52 are elongated, generally U-shapedrigid members that extend between spaced apart side edges 54 and 56 ofthe forcer 12. In particular, legs of the U-shaped members extend in asubstantially parallel relationship toward the platen 14 and terminatein respective pole faces (ends of the pole teeth) located adjacent tothe platen 14. The distance between the pole faces and the platen teethdefines an air gap 58 for the motor system 10. The air gap 58, forexample, can be maintained by a bearing support (e.g., low frictionbearings or air bearings) operative to permit substantially freemovement between of the forcer 12 and platen 14. The legs further definethe associated teeth 36, 38, 40. The poles 42-52, for example, areformed of a ferrous material or other suitable magnetizable material.

A central channel (or slot) 60, 62, 64, 66, 68, 70 also extendslongitudinally through each respective pole 42, 44, 46, 48, 50, 52. Eachchannel 60, 62, 64, 66, 68, 70 is dimensioned and configured to receivepart of an associated coil 72, 74, 76. Each coil 72, 74, 76 includes aplurality of windings of an electrically conductive material (e.g.,wire). Thus, the pole pair 30 and coil assembly 72 define phase A, thepole pair 32 and coil assembly 74 define phase B, and the pole pair 34and coil assembly 76 define phase C of the three phase system 10.

While the example of FIG. 1 employs pre-wound coil assemblies insertedwithin associated slots at each pole, those skilled in the art willunderstand and appreciate that coils 72, 74, and 76 could be wound insitu around corresponding parts of the poles 42-52. In addition, coilconfigurations different from that illustrated could be utilized to forma motor system 10, in accordance with an aspect of the presentinvention.

In accordance with an aspect of the present invention, each pole 42, 46,50 is magnetically coupled to its associated pole 44, 48, 52 of eachrespective pole pair. With particular reference to the pole pair 30, anelongated permanent magnet 80 is interposed between the poles 42 and 44.The magnet 80 extends longitudinally between the side edges 54 and 56 ofthe forcer 12. The magnet 80 may operatively hold the poles 42 and 44together in a desired relationship, such as due to the magnetic fieldproduced by the magnet. Other connecting means also could be employed tofix the poles at a desired orientation, such as illustrated in FIG. 1.The pole pairs 32 and 34 similarly include magnets 82 and 84 thatmagnetically couple the respective poles 46, 48 and 50, 52. Additionalretaining means (e.g., glue, bolts, etc.) also could be employed to holdeach pair of poles 42 and 44, 46 and 48, 50 and 52 together.

The permanent magnets facilitate the flow of magnetic flux through eachpole pair. By way of illustration, when the motor system 10 is in anunexcited condition, the permanent magnet flux flows into the U-shapedstructure of the pole and divides evenly between the two branchesthereof. The flux traverses the air gap at the pole faces between thepole teeth and the platen. It then flows through the platen, crosses theother air gap, and divides evenly between the pole faces of the secondU-shaped lamination to complete its circuit at the opposite end of thepermanent magnet.

When current is established in a winding, its generated flux reinforcesthe permanent magnet flux at one pole face and diminishes it at theother pole face. In this manner, the permanent magnet flux can beeffectively “commutated” from one pole face to the other. By varying thewinding current's sign and amplitude, the flux can be partially orcompletely directed to either pole face.

After the respective poles have been assembled, as shown in FIG. 1, asuitable encapsulation, indicated in phantom at 88, may be applied topermanently fix the respective poles at a desired position. Theencapsulation, for example, may be an epoxy or thermoplastic material.

The foregoing arrangement provides a three phase linear motor system 10that facilitates the use of conventional control electronics. Forexample, the motor system 10 may employ an encoder (e.g., magnetic,optical, etc.) to monitor the relative (or absolute) position of theforcer 12 relative to the platen 14 and provide an encoder signalindicative thereof. A motor controller, such as three phase servocontroller, can implement and control energization of the respectivecoils 72, 74, 76 based on the encoder signal to provide desired movementof the forcer 12 along the path provided by the platen 14. As a result,the motor system 10 is capable of accuracy comparable to a linearbrushless AC motor, but at a reduced manufacturing cost due to thereduction in the number of permanent magnets.

FIGS. 2-7 depict movement of the linear motor system 10 in accordancewith an aspect of the present invention, in which identical referencenumber are employed to refer to corresponding parts of the system 10previously shown and described with respect to FIG. 1. The followingfigures illustrate the forcer moving in a single direction, although itis to be understood and appreciated that the motor system 10 may beemployed to provide movement in any direction and direction combinationsin accordance with an aspect of the present invention. The coils 72, 74and 76 are described as being energized by respective phase currentsI_(a), I_(b), I_(c), which vary as a three phase signal according to apercentage of an available drive current. For example, the three phasesignal can be a square wave or sinusoidal in which two of the threephases are respectively 120° and 240° out of phase relative to the otherphase.

Turning to FIG. 2, the forcer 12 is oriented relative to the platen 14such that teeth 36 of the poles 40 and 42 are aligned with platen teeth18. In particular, one tooth 36 of each of the poles 42 and 44 isaligned with associated platen teeth 18 while the other teeth of thepoles are aligned with valleys between platen teeth. By way of example,at such a position, the excitation current (I_(a)) through the winding72 of phase A is at 100% of a desired drive current to provide a minimumreluctance condition at the aligned teeth. The electrical currents(I_(b), I_(c)) through the other phases B and C, in turn are set to −50%of the drive current.

FIG. 3 illustrates a motor condition corresponding to a motor conditionshown and described with respect to FIG. 2. In FIG. 3, magnetic fluxlines are depicted in each of the pole pairs 30, 32, and 34 according tothe relative position of the pole teeth 36, 38, and 40 and the platenteeth 18. The flux lines 90, 92, and 94 correspond to magnetic flux fromthe magnet through the respective pole pairs 30, 32, and 34. The fluxlines 96, 98, and 100 correspond to magnetic flux due to energization ofthe coils through the respective pole pairs 30, 32, and 34 at the levelsspecified with respect to FIG. 2. The platen 14 provides a return pathfor flux traveling through the poles 42-52. As a result of the alignmentbetween the teeth 36 of the pole pair 30 and the platen teeth 18,maximum magnetic flux lines 90 pass between the aligned teeth to providecorresponding normal force between the aligned teeth 36 and 18. Incontrast, the teeth 36 aligned with the valleys 22 of the platen 14 areillustrated as having no magnetic flux lines 96 traveling between theforcer 12 and the platen. Instead, the magnetic flux in such legs iseffectively canceled.

With respect to the pole pair 32, the magnetic flux passes through theend teeth 38 and associated platen teeth, which exhibit a level ofsubstantial alignment (e.g., greater than about 50% alignment)therebetween. The slight alignment between the middle pole teeth 38 andplaten teeth 18 defines a separate magnetic flux path for flux 92 fromthe magnet 82.

The pole pair 34 also includes magnetic flux paths determined accordingto the relative amount of alignment between its pole teeth 40 and theplaten teeth 18. As illustrated in FIG. 3, the middle teeth 40 andassociated platen teeth, which exhibit the location of substantial toothalignment for the pole pair 34, provide a magnetic flux lines 100 havingmaximum flux for the respective poles 50 and 52. In contrast, theoutermost teeth 40 of the pole pair 34, which are only slightly aligned(e.g., less than 50% alignment) with the platen teeth, provide a fluxpath having proportionally less magnetic flux lines 94 when compared tothe flux lines 100 passing through the intermediate part of the polepair 34.

In view of the description provided with respect to FIG. 3, thoseskilled in the art will understand and appreciate how the flux paths andmagnetic flux lines will vary according to the relative position of theforcer and platen, as described with respect to FIGS. 4-7.

Turning to FIG. 4, the forcer 12 has moved relative to the platen, suchthat no pole teeth 36, 38, 40 are aligned with the platen teeth 18.Specifically, the teeth 38 of the pole pair 32 for phase B arecompletely misaligned with associated platen teeth 18. Accordingly, noexcitation current I_(b) flows through the coil assembly of phase B. Incontrast, the electrical currents (I_(a), I_(b)) through the coilassemblies 72 and 76 for phases A and are energized to about 86.6% and−86.6% of the drive current, respectively.

FIG. 5 corresponds to a subsequent motor condition in which teeth 40 ofthe pole pair 34 are aligned with the platen teeth 18. Thus, theexcitation current through the coil 76 is set to −100% of the availabledrive current so as to achieve a minimum reluctance condition. Theexcitation current through the other coils 72 and 74 are set to about50% of the available drive current.

FIG. 6 illustrates a motor condition similar to FIG. 3 in which nocurrent is provided to the coil assembly 72 associated with phase A, dueto the misalignment between the teeth 36 of the pole pair 30 and theplaten teeth 18. An excitation current of about +86.6% of the drivecurrent is provided to the coil assembly 74 of phase B and about −86.6%of the drive current to the coil assembly 76 of phase C.

FIG. 7 shows a motor condition in which teeth 38 of phase the pole pair32 are aligned with associated platen teeth 18. Thus, at such condition,the excitation current applied to the coil assembly 74 is at +100% ofthe available drive current. The alignment between the teeth 38 and 18provides a path for maximum magnetic flux and, thus, a minimumreluctance condition. The current applied to the other coils 72 and 76associated with phases A and C are respectively at −50% and +50% of theavailable drive current.

Those skilled in the art will understand and appreciate various controlschemes that could be implemented with the three phase motor system 10in accordance with an aspect of the present invention to effect desiredmovement of the forcer 12 relative to the platen 14.

FIG. 8 illustrates an alternative configuration for a three phase linearmotor system 120 in accordance with an aspect of the present invention.In this example, the motor system includes a forcer 122 that is moveablerelative to platen 124. The platen has a plurality of teeth 126 spacedapart from each other according to a defined tooth pitch 128. Inaccordance with an aspect of the present invention the platen teeth 126are not permanent magnets. That is, the platen 124 is formed of anelectrically conductive material (e.g., iron) that is capable of beingmagnetized in response to magnetic fields generated in response toenergizing the forcer 122 so as to provide a return path for magneticflux from the forcer. In accordance with an aspect of the presentinvention, the platen teeth 126 are not permanent magnets.

The forcer 122 includes three pole pairs 130, 132, and 134. Each polepair 130, 132, 134 includes respective set of teeth 136, 138, 140, whichare arranged as a function of the platen tooth pitch 128. In particular,each set of teeth 136, 138, 140 is offset from an adjacent set of teethby about 120 degrees of the tooth pitch 128. In particular, the set ofteeth 138 of the pole pair 132 is offset by about 120 degrees from theset of teeth 136 of the pole pair 130. The set of teeth 140 of the polepair 134 offset 240 degrees of the tooth pitch relative to the set ofteeth 136 of the pole pair 130 and offset 120 degrees of the tooth pitchrelative to the set of teeth 128 of the pole pair 132. As a result, ofsuch teeth spacing, at most one set of teeth 136, 138, or 140 can becompletely aligned with platen teeth 126 at a given motor condition. Inthe motor condition illustrated in FIG. 8, two teeth of the pole pair130 are aligned with associated platen teeth 126 and the other two teethof the pole pair 130 are aligned with valleys of between platen teeth.

In the example of FIG. 8, each of the pole pairs 130, 132, 134 furtherincludes separate poles 142 and 144, 146 and 148, and 150 and 152,respectively. Each of the pole pairs 130, 132, and 134 further includesa coil 154, 156, and 158 disposed around part of the respective poles,such as extending through associated slots formed in each pole 142, 144,146, 148, 150, and 152. Each coil 154, 156, 158 includes a plurality ofwindings that define a respective phase A, B, C of three phase motorsystem 120. While a single coil is illustrated as being associated witheach phase, it is to be appreciated that more than one coil could beassociated with each phase, such as connected in series or in parallel.

In accordance with an aspect of the present invention, the associatedpoles 142 and 144, 146 and 148, and 150 and 152 of each respective polepair 130, 132. 134 arc magnetically coupled to each other. In theexample of FIG. 8, the magnetic couplings are implemented by anarrangement of permanent magnets 154 and 156, 158 and 160, 162 and 164located at the distal end of each pole, which magnets are electricallyconnected together by a plate of an electrically conductive material(e.g., back iron) 166, 168, 170. Each magnet pair 154 and 156, 158 and160, 162 and 164 of each associated pole pair 130, 132, 134 has anopposite polarity so that the combination of magnets and back ironprovides a flux path through which magnetic flux lines can travel. Whilea pair of oppositely polarized permanent magnets 154 and 156, 158 and160, 162 and 164 are illustrated at the distal ends of the associatedpoles, it is to be appreciated that a suitable magnetic coupling may beprovided by one of the pieces being a permanent magnet and the otherpiece being a magnetizable material, such as iron.

In view of the foregoing those skilled in the art will understand andappreciate that a three phase linear motor system, in accordance withthe present invention, can embody various motor configurations similarto linear stepper motors. FIGS. 9 and 10 illustrate possible alternativemotor configurations that can be implemented for a three phase linearmotor system in accordance with an aspect of the present invention. Forsake of brevity only a single phase of such three phase motor systemsare depicted in FIGS. 9 and 10. It is to be understood and appreciated,however, that other phases of the respective three phase motor systemswould be substantially identical to the illustrated phase, but be offsetso that at most one set of pole teeth (of an associated phase) can alignwith the platen teeth at a given time.

FIG. 9 illustrates part of a motor system 200 in which part of a forcer202 is spaced apart from a platen 204 by an air gap 206. The platen 204includes teeth 208 separated by valleys according to a tooth pitch 210.In accordance with an aspect of the present invention, the teeth 208 donot include permanent magnets, but are selectively magnetizable inresponse to an electric field generated by energizing the motor system200. The platen 204 thus provides a return path for magnetic fluxtraveling through the forcer 202.

The forcer 202 includes three pole pairs, including the illustrated polepair 212 that includes two magnetically coupled poles 214 and 216. Inthis example each pole 214, 216 is illustrated as a generally U-shapedelement having legs that define teeth 218 of the respective poles. Thepole teeth 218 have a width that is substantially the same as the widthof the platen teeth 208 and have a tooth pitch that approximates aninteger multiple of the tooth pitch 210. As a result, when some of thepole teeth 218 are aligned with platen teeth 208, the other pole teethof that pole are aligned with valleys of the platen.

In the example of FIG. 9 each pole 214, 216 includes a separate coil220, 222 associated therewith. The coils 220 and 222 may be connected inseries or in parallel to define a phase of the three phase linear motorin accordance with an aspect of the present invention. In addition, thepoles 214 and 216 are magnetically coupled. By way of illustration, thepoles 214 and 216 are magnetically coupled by a permanent magnet 224interposed between adjacent side edges of the poles, which permanentmagnet has a width equal to the tooth pitch 210. The permanent magnet224 provides a flux path for magnetic flux lines to travel betweenpoles, including when the associated phase coils 220 and 222 areenergized. Alternatively, the poles could be magnetically coupled by amagnet and back iron arrangement similar to that shown and describedwith respect to FIG. 8.

Those skilled in the art will understand and appreciate that the othertwo pole pairs (not shown for sake of brevity) of the three phase linearmotor system 200 will be similarly configured with the pole pair 212,but offset from the pole pair 212 by +/−120 degrees of the tooth pitch210 and by +/−240 degrees of the tooth pitch, respectively. As a resultof such motor configuration, the teeth of at most one of the pole pairswill be aligned with the platen teeth. By such configuration, the motorsystem 200 also is well suited to be controlled by a standard threephase servo motor controller.

FIG. 10 illustrates yet another example of a three phase linear motorsystem 250 in accordance with an aspect of the present invention. Themotor system 250 includes a platen 252 that provides a path over whichone or more forcers 254 may travel. The platen 252 includes a pluralityof teeth 256 that extend in a substantially parallel manner from aplanar surface of the platen. The teeth 256 are spaced apart from eachother in a direction of movement 258 by valleys to provide a definedtooth pitch 260. In accordance with an aspect of the present invention,the teeth 256 are not permanent magnets, but are formed of amagnetizable material. Thus, the platen 252 provides a return path formagnet flux generated at the forcer 254.

The forcer 252 includes three pole pairs, one of which, indicated at262, is depicted in FIG. 10. The pole pair 262 includes a pair of poles264 and 266. In the illustrated example, each pole 264, 266 includes apair of slots dimensioned and configured to receive a respective coil268, 270. The coils 268 and 270 may be pre-wound assemblies or be woundin situ around the respective pole pieces, as is known in the art. Thecoils 268 and 270 are electrically connected to each other, such as inseries or parallel, to provide a common phase of the three phase linearmotor system 250. Each of the pole 264, 266 further includes fourrespective teeth 272, 274 that extend toward the platen 254 in anopposing relationship relative to the platen teeth 256. The teeth 272and 274 have a width (in the direction of movement 258) that issubstantially identical to the tooth width of the platen teeth and arearranged to approximate the tooth pitch 260 at the platen.

The platen teeth 256 and pole teeth 272 and 274, for example, can have awidth from about 5 mm to about 15 mm. This facilitates manufacturing ofthe motor system according to the present invention, as the teeth may beformed by machining (e.g., cutting or grinding) in contrast to moreexpensive etching techniques typically employed for smaller tooth widths(e.g., 1 mm or less). Such manufacturing specifications are furtherenabled by implementing standard three phase servo motor controls with apositioning encoder to control operation of the motor system 250.

In accordance with another aspect of the present invention, the poles264 and 266 are magnetically coupled. By way of illustration, the poles264 and 266 are magnetically coupled by a permanent magnet 278interposed between and interconnecting adjacent side edges of the poles.The magnet 278 extends a width between the poles commensurate with theplaten tooth pitch. The permanent magnet 278 provides a path thatfacilitates the travel of magnetic flux lines between poles 264 and 266,such as when the associated phase coils 268 and 270 are energized by anexcitation current. Alternatively, the poles 264 and 266 could bemagnetically coupled by a magnet and back iron arrangement similar tothat shown and described with respect to FIG. 8.

The other pole pairs of the three phase motor system 250 are configuredsubstantially identically to the pole pair 262. However, the teeth ofthe other pole pairs are offset in a direction of movement 258 relativeto the set of teeth 272, 274 of the pole pair 262 by +/−120 degrees and+/−240 degrees of the platen tooth pitch 260, respectively. In this way,at most, one set of pole teeth can be aligned with the platen teeth 256at a given time. The motor system 250, similar to the other arrangementsshown and described herein, can be controlled by a standard three phaseservo motor controller. By employing a three phase motor controller, thetooth width may be provided at a dimension that is easily machined(e.g., ranging from about 5 mm to about 15 mm). Reducing the number ofpermanent magnets also reduces manufacturing costs, as the platen teeth256 are not permanent magnets, as is typical in three phase linearmotors.

While the foregoing examples have shown and described three phase linearmotors, in accordance with the present invention, that travel in asingle dimension, those skilled in the art will understand andappreciate that the principles contained herein may be extended totwo-dimensional. FIG. 11 illustrates an example of a two-dimensionallinear motor system 300, which may be implemented in accordance with thepresent invention. The motor system 300 includes a stage 302 that ismoveable in both the X and Y directions relative to a platen 304. Thestage 302 is illustrated as having four three phase forcers 306, 308,310, and 312, although any number of forcers for each direction could beemployed in accordance with the present invention. Two of the forcers306 and 310 are oriented to move the stage in the Y-direction whenenergized and the other forcers 308 and 312 are operative to urge thestage in the X-direction when energized. Each of the forcers 306, 308,310, and 312 is configured as a three phase linear motor in accordancewith an aspect of the present invention (See. e.g., FIGS. 1-10).

While the platen 304 is illustrated as being generally flat, it will beunderstood and appreciated by those skilled in the art that the platencould be curved or have a generally tubular configuration. In addition,the coil and magnet assembly could be stationary and the platen could bethe moving member.

The platen 304 includes a plurality of spaced apart teeth 314 having agenerally square cross section that protrude from a planar surface ofthe platen. The teeth are separated by grooves 316 and 318 extending inthe X and Y directions. According to an aspect of the present invention,the platen teeth 314 are not permanent magnets, but instead areselectively magnetizable according to the electric fields generated inthe stage (or stages) 302 traveling thereon. That is, the platen teeth314 are formed of a material having a desired relatively high magneticpermeability, such as iron, so that the teeth and platen base from whichthey extend are easily magnetizable in response fields imposed from theforcers 306, 308, 310, and 312.

As a result, the teeth 314 and grooves 316 and 318 cooperate with theelectromagnet field generating mechanisms in the stage 302 (e.g., theforcers 306, 308, 310, and 312) to selectively move the stage in the Xand/or Y directions. As described above, each of the forcers 306, 308,310, and 312 is well suited for control by a standard three phase servomotor controller. In order to provide suitable position encoding toimplement the servo motor controls, a glass scale or optical encodingscheme may be employed to provide desired closed loop position feedback.Because the teeth are not formed of permanent magnets, manufacturingcosts can be significantly reduced when compared relative to existingthree phase linear motor systems. Manufacturing is further facilitatedbecause the three phase controls enable generally larger tooth sizes tobe implemented in accordance with an aspect of the present invention.Accordingly, more expensive chemical etching of platen teeth is notrequired to provide a desired high resolution linear motor system inaccordance with the present invention.

What has been described above includes exemplary implementations of thepresent invention. It is, of course, not possible to describe everyconceivable combination of components or methodologies for purposes ofdescribing the present invention, but one of ordinary skill in the artwill recognize that many further combinations and permutations of thepresent invention are possible. Accordingly, the present invention isintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.

What is claimed is:
 1. A forcer for a three phase servo motor system foruse with a platen having teeth of a non-permanent magnetic material,comprising: three pole pairs, each of the three pole pairs including atleast two operatively connected poles that are magnetically coupledtogether; a coil operatively associated with each of the three polepairs, each coil being operative to provide an electromagnetic field atthe respective pole pair when energized; and a set of teeth associatedwith each of the three pole pairs, the set of teeth associated with asecond pole pair of the three pole pairs being offset relative the setof teeth associated with a first pole pair of the three pole pairs byabout (+/−120°+L*180°)*P/360°, where L is a natural number and P is thetooth pitch, and the set of teeth associated with a third pole pair ofthe three pole pairs being offset relative the set of teeth for thefirst pole by about (+/−240°+M*180°)*P/360°, where M is a naturalnumber.
 2. The forcer of claim 1, the teeth of each of the pole pairshaving a tooth width greater than about 5 mm.
 3. The forcer of claim 1,the tooth pitch being about twice the tooth width.
 4. The forcer ofclaim 1, further comprising a permanent magnet interposed between atleast two poles of each of the three pole pairs to magnetically couplethe at least two poles of each respective pole pair.
 5. The forcer ofclaim 1, each of the three pole pairs further comprising: a permanentmagnet attached at an end of at least one pole of each of the three polepairs at a side thereof opposite the teeth thereof; and an electricallyconductive plate interconnecting the permanent magnet and the other poleof each respective pole pair, whereby the permanent magnet and theelectrically conductive plate magnetically couple the at least two polesof each respective pole pair.
 6. The forcer of claim 1 in combinationwith a platen to provide a linear motor system, the combinationcomprising: the platen including a plurality of platen teeth of amagnetizable material, the platen teeth being arranged in a spaced apartrelationship along a relative direction of travel between the forcer andthe platen to define the tooth pitch and provide a return path formagnet flux generated at the forcer; and the teeth of the three polepairs of the forcer being spaced in an opposing relationship apart fromthe platen teeth by an air gap to facilitate passage of flux betweengenerally aligned teeth when appropriate coils are energized.
 7. Theforcer of claim 6 in combination with a platen to provide a linear motorsystem, the combination comprising: the platen that includes a pluralityof platen teeth of a magnetizable material so as to provide a returnpath for flux generated at the forcer, the platen teeth being arrangedalong the direction of movement in a spaced apart relationshipcorresponding to a distance defined by the platen tooth pitch; and theteeth of the first, second and third pole pairs of the forcer beingspaced apart from the platen teeth by an air gap to permit magnetic fluxto pass between platen teeth substantially aligned with teeth of thefirst, second and third pole pairs based on three phase electric currentapplied to the windings of the respective first, second and third polepairs.
 8. The combination of claim 7, the forcer being moveable relativeto the platen along at least two orthogonal axes.
 9. The system of claim7, each of the first, second, and third pole portions further comprisinga pair of operatively connected poles that are magnetically coupledtogether.
 10. The system of claim 9, each pole further comprisingincluding at least one elongated channel extending between opposed sideedges of the pole to define at least two teeth of each respective pole,part of an associated coil being received within the channel each of thepole, such that an intermediate part of the respective pole isinterposed between different parts of each coil.
 11. The system of claim10, each of the platen teeth having a tooth width that is substantiallyidentical to the teeth of the first, second, and third pole portions.12. The system of claim 11, the tooth width of the platen teeth and theteeth of the first, second, and third pole portions being greater thanor equal to about 3 mm.
 13. The system of claim 11, the tooth width ofthe platen teeth and the teeth of the first, second, and third poleportions being greater than or equal to about 7 mm.
 14. The system ofclaim 11, the tooth pitch P being about twice the tooth width.
 15. Aforcer for a three phase servo motor system, comprising: a first polepair including a plurality of windings and a set of at least two teeth;a second pole pair including a plurality of windings and a set of atleast two teeth offset in a direction of movement from the set of teethof the first pole pair by about (+/−120°+L*180°)*P/360°, where L is anatural number and P is a tooth pitch of an associated platen; and athird pole pair including a plurality of windings and a set of at leasttwo teeth, the set of teeth of the third pole pair being offset in thedirection of movement about (+/−120°+M*180°)*P/360°, where M is anatural number, relative to the set of teeth of the second pole pair andoffset in the direction of movement about (+/−240°+N*180°)*P/360°, whereN is a natural number, relative to the set of teeth of the first polepair.
 16. A three phase linear motor system, comprising: a platen havinga plurality of teeth arranged in a direction of travel according to atooth pitch P, the platen teeth being formed of a generallynon-permanent magnetic material to facilitate selective magnetizationthereof; a forcer that is moveable relative to the platen in thedirection of travel, the forcer comprising: first, second, and thirdpole portions arranged in a spaced apart relationship along thedirection of travel, each of the first, second, and third pole portionshaving a set of at least two teeth oriented to oppose the platen teethand having a pitch functionally related to the tooth pitch P of theplaten teeth; the set of teeth of the second pole portion being spacedin the direction of travel relative to the set of teeth of the firstpole portion a distance d2 defined by d2=(+/−120°+L*180°)*P/360°, whereL is a natural number;  and the set of teeth of the third pole portionbeing spaced in the direction of travel relative to the set of teeth ofthe first pole portion a distance d3 defined byd3=(+/−240°+M*180°)*P/360°, where M is a natural number.
 17. The systemof claim 16, each of the first, second, and third pole portions furthercomprising at least two operatively connected poles that aremagnetically coupled together to facilitate passage of magnetic fluxtherebetween.
 18. The system of claim 17, further comprising a permanentmagnet interposed between associated poles of each of the first, second,and third pole portions to magnetically couple the at least two poles ofeach respective pole portion.
 19. The system of claim 17, furthercomprising: a permanent magnet attached at an end of at least one poleof each of the first, second, and third pole portions at a side of therespective at least one pole opposite from the teeth of each respectivepole portion; and an electrically conductive plate electricallyconnecting the permanent magnet and the other pole of the respectivepole portion, whereby the permanent magnet and the electricallyconductive plate magnetically couple the at least two poles of eachrespective pole portion.
 20. The system of claim 16, wherein the forceris moveable relative to the platen in at least two orthogonaldirections.
 21. A three phase linear motor system, comprising: a platenhaving a plurality of spaced apart teeth arranged in a direction oftravel according to a predefined tooth pitch, the platen teeth beingformed of a non-permanent magnetic material that is selectivelymagnetizable; and a forcer associated with a housing supported formovement relative to the platen along the direction of travel, theforcer comprising: a first pole pair formed of at least two magneticallycoupled poles and including a set of at least two teeth and a pluralityof windings disposed around part of the first pole pair; a second polepair formed of at least two magnetically coupled poles and including aset of at least two teeth and a plurality of windings disposed aroundpart of the second pole pair, the set of teeth of the second pole beingoffset in the direction of travel relative to the set of teeth of thefirst pole pair about (+/−120°+L*180°)*P/360°, where L is a naturalnumber and P is the predefined tooth pitch; and a third pole pair formedof at least two magnetically coupled poles and including a set of atleast one tooth and a plurality of windings disposed around part of thethird pole pair, the set of at least one tooth of the third pole pairbeing offset in the direction of travel relative to the set of teeth ofthe second pole pair about (+/−120°+M*180°)*P/360°, where M is a naturalnumber and being offset relative to the set of teeth of the first polepair about (+/−240°+N*180°)*P/360°, where N is a natural number.
 22. Athree phase variable reluctance linear motor system, comprising:selectively magnetizable toothed means for providing a magnetic fluxpath, the toothed means having a plurality of spaced apart teetharranged according to a defined tooth pitch for providing a travel path;moveable means positioned for movement along the travel path relative tothe toothed means, the moveable means comprising: first, second, andthird pole means arranged in a spaced apart relationship along the pathfor, when energized, generating a magnetic field that passes from theenergized electromagnetic means to associated teeth of the toothed meansgenerally aligned with associated teeth of the energized pole means,each of the first, second, and third pole means having a set of at leasttwo teeth oriented to oppose the associated teeth of the toothed meansand having a tooth pitch commensurate with the defined tooth pitch; theset of at least two teeth of the second pole means being offset alongthe travel path relative to the set of teeth of the first pole means adistance equal to about (+/−120°+L*180°)*P/360°, where L is a naturalnumber and P is the defined tooth pitch; and the set of at least twoteeth of the third pole means being offset along the travel pathrelative to the set of teeth of the first pole means a distance equal toabout (+/−240°+M*180°)*P/360°, where M is a natural number.
 23. Thesystem of claim 22, each of the first, second, and third pole meansfurther comprises separate pole portions, the system comprising meansfor magnetically coupling the separate pole portions of each of thefirst, second, and third pole means.
 24. The system of claim 22, thetravel path including at least two orthogonal directions.